A thermal flowmeter is known as a typical flow measuring device having a sensing element for outputting a nonlinear signal in accordance with a flow rate. A thermal flowmeter can directly sense, for example, a mass flow rate of intake air to be fed into the internal combustion engine of an automobile. The measured flow rate is used as computation data for fuel injection control of an electronic control type.
A heat resistive element used for the thermal air flowmeter has a temperature dependency. Examples thereof are a hot wire type heat resistive element produced by means of winding a platinum wire around a bobbin and of coating the platinum wire with glass, and a thin film type heat resistive element produced by forming a thin film resistor on a ceramic substrate or a silicon substrate.
As systems for measuring a flow rate, the following systems are proposed. One of them is a system of measuring an air flow rate by converting a heating current flowing through the heat resistive element into a voltage, wherein the heating current is controlled so as to maintain a constant temperature difference between a temperature of the heat resistive element whose heat is taken away in accordance with the air flow rate and a temperature of air to be measured as flow rate. Another is a system of measuring an air flow rate based on a temperature deference between temperature sensitive elements (thermo-sensitive resistors) disposed on both sides of a heat resistive element in a direction of air flow.
Air flowing through an intake pipe of an engine pulsates due to opening and closing operations of an inlet valve of the engine and hence an output signal of a flow sensor (thermal flowmeter) also accompanies pulsation. Incidentally, a rate of change in output voltage of the thermal flowmeter is large (high sensitivity) in a low flow rate region and the rate of the change changes slowly as the flow rate increases (low sensitivity). That is, a thermal flowmeter shows a so-called nonlinear output characteristic.
In the nonlinear output characteristic, a voltage of the flow sensor can be represented by the fourth root of a flow rate (King's formula). The output characteristic Qref is shown in FIG. 20. The vertical axis represents a sensor output voltage (V) and the horizontal axis represents an air flow rate. In FIG. 20, as an example, a sensor output voltage Vin to a flow rate Qa with relatively large pulsation (ripple) and a sensor output voltage Vinf to a flow rate Qaf with relatively small pulsation (ripple) are shown. Even when an actual pulsing flow rate has a curve like a sine curve, the sensor output voltage has a somewhat distorted waveform whose positive-side is compressed and whose negative-side is extended. If the output voltage containing such pulsative component is rendered as a mean value (average) as-is status, the mean value of flow rate-waveform apparently decreases (as an error). The error increases as the pulsation increases. In FIG. 20 for example, whereas an actual mean value of a sensor output voltage Vin should be Vave1, the apparent mean value is Vave2, and that causes the error Vave1−Vave2. Note that, when pulsation is significantly increased until back-flow occurs, the mean value of flow rates apparently increases in reverse to the above-mentioned. Especially, in the case of low rotation speed or heavy loading operation of an engine having four or fewer cylinders, it sometimes happens that the amplitude of the pulsation of an intake air flow rate is large and back-flow is partially accompanied. In such a case, a pulsation error (ripple error) of the sensed flow rate signal is apparently caused in the positive (namely flow rate increase) direction. The pulsation error causes measurement accuracy to lower. Here, the relationship between the degree of pulsation (a ripple rate) and the error of a sensor output is described later in reference to FIGS. 8 and 10.
A system of using a variable filter in order to reduce such a pulsation error is described in JP-A No. 161122/2000.
Incidentally, as a conventional art for reducing a pulsation error, a technology of linearizing a sensor output voltage is known. Generally, data used such a linearization processing is made and stored by the following method: namely, measuring a voltage signal of a sensor related to a flow rate actually; making a characteristic curve used as reference (called a master characteristic Qref) based on the actual measured voltage and the flow rate; and storing the characteristic curve data in an engine control unit or the like. Then, the linearization processing (V-Q conversion processing) is carried out by applying the master characteristic Qref to the real flow rate sensed by the sensor.
The pulsative component of a sensor output voltage includes also an error component such as noise. A system of smoothening a signal with a hard filter in advance of the linearization processing and thereafter applying the linearization processing in order to reduce such an error component is known.
In the system, as shown in FIG. 21, an output voltage value (a signal sensed by a heat resistive element) is smoothened so that the amplitude thereof is reduced (for example, so that an amplitude V1 is reduced to an amplitude V2) while the mean value itself of the output voltage is not changed. However, when a sensed signal is linearized (V-Q conversion) after filtered (smoothened) as stated above, an error arises also in the mean value of a converted flow rate, as shown in FIG. 21.
In order to reduce such an error, JP-A Nos. 316145/1999 and 337382/1999 propose a system of smoothening a sensor output voltage with a filter after linearizing (V-Q conversion) the sensor output voltage. Then, a flow rate signal whose pulsation (ripple) amplitude is reduced with the filter is nonlinearized again (Q-V conversion), subjected to D-A conversion, and input into an engine control unit which has a function for the linearization processing.
JP-A No. 20454/2004 proposes a system of modulating the output power of a nonlinear output signal of a sensor with a parameter for modulation, and thereafter applying unequal linearization for modulating a mean value. By the method, it is possible to arbitrarily modulate the mean value in accordance with the magnitude of pulsation and to obtain an air flow rate signal with a higher degree of accuracy.